Twist drill



June 29, 1943. G. sTEvEN 7 2,322,894

TWIST DRILL Filed June 29, 1939 [Ml 5 7-0 6602 8 62a Wa 1;

Patented June 29, 1943 UNITED TWIST DRILL George Stevens, Baltimore, Md., assignor to Rustless Iron and Steel Corporation, a corporation of Delaware Application June 29, 1939, Serial No. 281,944

1 Claim.

My invention relates to the drilling of tough metals and other materials, especially to the drilling of the tough, work-hardening, austenitic chromium-nickel stainless steels, and to the drill employed in the same.

The general object of my invention is the drilling of tough metals, such as various austenitic alloy irons and steels, in a simple, efiicient and directmanner giving a cleaner and more uniform drilled surface than in heretofore known drilling operations on the same.

Another object is the production of a twist drill capable of drilling tough and/or work-hardening alloy steels more quickly and at higher speeds than with heretofore known drills, all with a greater length of drill life and with a minimum of building up of metal at the cutting edges and heating at drill point, and with a minimum of drill failure through burning on of chips.

A further object of my invention is to evolve a mode of renewing the cutting edge of the drill, when the latter has become worn in operation, which will, in very simple and rapid manner, most advantageously duplicate the original cutting edge and utilize the entire useful length of the drill.

Other and further objects of my invention will in part be apparent and in part pointed out hereinafter in connection with the following more detailed description of a preferred embodiment of my invention.

The invention accordingly consists in the combination of elements, features of construction and arrangement of parts, and in the several operational steps and the relation of each of the same to one. or more of the others, all as described herein, the scope of the application of which is indicated in the claim at the end of the speci fication.

In the accompanying drawing,

Figure 1 is a fragmentary elevation view of a twist drill embodying certain features of my invention.

Figure 2 is a view of the drill of Figure 1 as seen from the right.

Figure 3 is a fragmentary sectional view of the drill as seen on the plane formed by intersecting lines l--l and 2-2 of Figure 1.

Like reference characters denote like parts throughout the several views of the drawing.

As conducive to a clearer understanding of my invention, it may be pointed out at this time that. in the past considerable difiiculty hasbeen experienced in the machining of certain tough metals, particularly certain alloy steels which are inclined to work-harden. Perhaps the austenitic chromium-nickel stainless steels are among the most common in this regard. These may contain 18% chromium, 8% nickel, and the balance iron. Similar difficulties are encountered in some of the lower chromium-nickel steels, the high manganese steels and the high nickel-high manganese steels. Hitherto compromises have frequently been resorted to, such, for example, as the introduction of sulphur, phosphorus, selenium or tellurium in the chromium-nickel steels to impart free-machining qualities by giving metal which forms brittle chips. The introduction of such ingredients, however, is attended by a sacrifice of other important properties and by an increase in cost.

Heretofore, and prior to my invention, the twist drills employed in practice for reasons which I will more fully point out hereinafter, have had cutting edges which are comparatively blunt. These blunt edges have been found to drill into the work piece of metal such as I have described in the foregoing by what I term a compressing and scraping action. Such action appears to be entirely satisfactory for metal giving brittle chips, such as Bessemer screw stock and time to time leaves with the chip. Moreover, the

local high temperatures resulting from a drilling action of this kind frequently have been found to cause the built-up cutting edge to adhere to the i edge of the drill, an action which decreases drill efficiency, increases torque requirements and ultimately precludes further drilling action.

As a further point, where the drilling action is in the nature of the usual compressing and scraping, the metal at the drill point hardens to an exceptional degree by virtue of the work-hardening characteristics of the austenitic steels. course, further increases the difliculty of drilling the metal.

An object of my invention, therefore, is to avoid the, aforementioned disadvantages of prior practices and to provide a drill and achieve a method of drilling tough, work-hardening metals to give a minimumof work-hardening, a reduc This, of

more lands and flutes, I prefer to form it with two since it is particularly adapted for end-cutting into solid metal. Where side cutting into metals already having a bore therethrough is desired, a drill having three or more lands, of course, may be employed.

Lands IB'and II are separated by flutes l2 and I3. Lands and flutes are of gradual twist or pitch, extending helically throughout the length of the drill. These helical flutings serve as channels for clearing chips from the region where the drilling action takes place. It is apparent that if these chips were not removed, they would clog and bind in the region of drilling, torque requirements would build up, and the drill would shortly fail at some point along its length. To facilitate the drilling action, the leading edge of each land is provided with the usual clearance bevels Illa and Ila.

The drill terminates in a chisel end l4 and. cutting faces or lips Iilb and Ill). These respectively terminate in cutting edges Ito and Ho. Referring more particularly to cutting edge We, it is apparent that this cutting edge is defined by the junction of the leading surface llld of land W with face lilb. This best may be seen in Figure 3. Now, since the angle A defined between these two elements is comparatively large, the cutting edge thus produced is comparatively blunt. In operation, such a cutting edge will rub or chafe against the work piece, removing metal therefrom by a compressing and scraping action. This action in the drilling of tough meta1 results in the production of the detrimental heat, increased torque and rapid wear of the drill, as aforesaid.

In my consideration of this problem, I observed that if this angle A could be materially reduced, a sharper cutting edge would result, which, if properly applied against the work piece, probably would produce the drilling action by cutting, rather than by scraping or grinding. This then would result in a diminution of heat and an in crease in the life of the drill-itself. The difficulty which presented itself, however, was how to diminish this angle A without disturbing the other factors of drill design which are so essential to the production of a drill capable of proper functioning. It was readily apparent that the clearance angle of the cutting faces could not be varied materially. This is because of the fact that if the lips have too much clearance, that is, too great a clearance angle, the drill bites too deeply into the metal and causes abrupt failure of the cutting edge. Ordinarily the clearance angle amounts to from 12 to 15 as shown in Figure 3. Thus it is impossible to greatly alter theangle of the cutting faces Iflb and Ill),

On the other hand, if attempt were made to decrease the angle of junction of lip and land as defining the cutting edges, this could be accomplished only by tightening materially the twist or pitch of the lands and flutings of the drill. Thus, while this expedient would improve the cutting action appreciably, nevertheless, a drill so modified would weaken the cutting edge and also make it impossible for the chips to clear themselves through the tightened twist of the drill. They would jam and bind between the drill and the work piece, and quickly result in build-up of torque requirements, followed by failure of the drill in shear. Thus, this possible expedient must be dismissed as unsatisfactory.

To avoid this impasse, I employ the novel expedient of providing a plurality of grooves along the leading wall of each land, these grooves being of small diameter and extending substantially side by side and parallel to the corresponding cutting edges of the drill. For land It, see Figures 1 and 2, the parallel grooves are shown at we with portions of the leading wall I 0d of the land intervening. In a typical instance, as for a inch drill, the grooves are 1% of an inch in diameter while the intervening portions of the wall are s g of an inch in width. It will be understood, of course, that the widths and depths of grooves and intervening lead wall portions may be changed as desired depending upon the drill size and other factors. As a matter of convenience,'I elect to call these grooves chip-curler grooves, because, being substantially semi-circular in contour, the leading groove, when viewed r in the direction of the cutting edge, has the property of imparting a curl to the chips, all as will be more fully pointed out hereinafter,

The cutting faces "lb and Nb of the drill are ground back so that the cutting edges I00 and H0 are defined by thejunction of these faces with the leading edges of the lowermost chip curler grooves. It will be evident from a consideration of Figure 3 that the dihedral angle defined between the cutting face Nb and the tangent to the leading edge of the lowermost groove Hie is quite small, so that a sharp cutting edge Hlc is automatically produced.

The cutting edges We and He are found to bite into the work piece, producing the drilling action by cutting rather than by grinding or scraping. The hooked end of the chipcurler groove, forming part of the cutting edge, in-- creases the cutting angle and helps pull the metal of the work piece, as the cutting action is carried on. The chip is spread away from the work piece as it rides up onto the curvature of the chip-curler groove, thus dispersing the heat from the cutting edge. This separating action is believed to be an exceedingly important aspect of my invention. The chip formed is continuous.

The cut in the work piece is smooth. There is no building up of metal on the cutting edges of the drill. Torque requirements are reduced. Increased drill speeds are possible. An increase in drill life also follows.

In spite of the fact that the chisel edge l4 functions primarily by grinding or pulverizing action, my new drill, considered in its entirety, is found to function essentially by cutting, the chips, once severed from the work piece, curling readily in tight spirals as they are pushed first from the trough of the lowermost chip-curler groove of each lip into the drill flutes from whenc they are conveyed to the exterior of the drilled hole and exhausted.

I have found that during operation, my new drill is capable of machining alloy steels of great toughness and/ or high work-hardening qualities,

without undue wear of the drill, and without the production of detrimental speeds are permissible, and the cutting action, as

contrasted with scraping or grinding action permits functioning with diminished torque requirements.

heat. Increased.

The very real utility of my. invention may be more fully grasped from certain comparative experimental data obtained in drilling a typical work-hardening alloy steel analyzing 18% chromium, 8% nickel, and the balance iron. A work piece of this metal was drilled, first with the conventional twist drill, and then with the new drill according to my invention. In each instance,the drill speed was eighty surface feet per minute, and in each instance the thickness of the work piec traversed was the same. Operating conditions were identical in the two cases. It was found that while the standard twist drill failed, and had to be reground, after only 17 pieces had been machined, my new drill machined 80 pieces without drill failure. Thus my new drill, making use in practical manner of a novel cutting principle, in the drilling of tough, work-hardening, alloy steels, is proved to have practically five times the wearing qualities of the standard twist drill.

It has additionally been found during the course of my investigations that under identical conditions, the rate of cutting, is greater in the case of my new drill than is possible with the conventional twist drill. In a typical instance, for example, my new chiD-chrler drill bored to a prescribed depth in 32 seconds, while a full 44 seconds were required to reach the same depth, employing a standard twist drill. In the cited case, therefore, my new drill was found to be about 38% faster than the conventional twist drill. At the same time, the fact that the drill penetrates by cutting action rather than by scraping or grinding results in the drill requiring less horse power due to its lower torque and axial point pressure than does the conventional twist drill.

Of particular novelty and interest is the simplicity with which the cutting edge of my new tool, when dulled, can be renewed to a substantial duplicate of the original cutting edge. Ordinarily, when employing the conventional twist drill, it is often necessary to use mechanical means for guiding the drill in renewing the cutting edges. This is necessary because of the difficulty in manually grinding back the cutting faces while maintaining the rather critical point angle unchanged. In my drill, however, the cutting edges are simply ground back to the leading edges of the neXt adjacent chip-curler grooves, the chip-curlers serving as guides to the proper point angle. Inasmuch as in a typical instance the width of the land which must be ground back is only about a of an inch, little more than say 1% of an inch must be ground away at each renewal of the cutting edge.

While my invention may be advantageously employed on twist drills of practically all sizes, it is particularly adapted to drills of comparatively large size. With drills of 1 a inch or less diameter, it is generally impractical to provide chip-curler grooves along the land walls.

Thus it will be seen that by the exercise of my invention, there is produced a drill which has marked advantages over the twist drills hitherto known. It has a greater life and moreover it is capable of a better cutting action than is possible with the conventional twist drill. In addition, it is seen that the drilled surface is smoother and more uniform than that obtained in conventional drilling practice.

As other embodiments may be made of my invention, and as changes may be made in the embodiment hereinbefore set forth, it will be understood that all matter described herein, or shown in the accompanying drawing, is to be interpreted as illustrative, and not in a limiting sense.

Having explained the nature and objects of my invention and having specifically described a preferred constructional embodiment thereof, what I claim as my invention is:

A twist drill for drilling tough metals and materials, said drill including a multiplicity of like grooves evenly spaced along the leading walls of the drill lands in a substantially parallel relationship, with the grooves along each of said walls substantially parallel to the plane of the cutting face associated with the respective land, the groove of each of said walls nearest the drill tip defining a cutting edge with the drill face.

GEORGE STEVENS. 

